Recent development of information equipment is remarkable, and demand for thin displays having high resolution and low power consumption is increasing with research and development advancing. Amongst these, the liquid crystal display for which the optical properties can be changed by electrically controlling the arrangement of the liquid crystal molecules, is anticipated as a display that can correspond to the above needs. As one form of such a liquid crystal display, a projection type liquid crystal display (liquid crystal projector) is known, which enlarges and projects an image emitted from an optical system using a liquid crystal light valve, onto a screen through a projection lens.
The projection type liquid crystal display uses a liquid crystal light valve as an optical modulation device, but a projection type display which uses a digital mirror device (hereinafter referred to as DMD) as the optical modulation device, instead of the liquid crystal light valve, has been put to practical use. However, this type of conventional projection type display has problems as described below.    (1) Due to leakage of light and stray light, which occur in various optical elements constituting an optical system, sufficient contrast cannot be obtained. Therefore, the gradient range (dynamic range) which can be displayed is narrow, and the image quality and power is inferior, as compared with an existing television receiver using a cathode ray tube (hereinafter referred to as CRT).    (2) Even if it is attempted to improve the image quality by various kinds of image signal processing, since the dynamic range is fixed, a sufficient effect cannot be demonstrated.
As a solution for the problems of the projection type display, that is, as a method of extending the dynamic range, it can be considered to change the amount of light shone onto the optical modulation device (light valve) corresponding to the image signal. The simplest method for realizing this is to change the optical output intensity of the lamp. In the projection type liquid crystal display, a method of controlling the output light of a metal halide lamp is disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 3-179886.
As a lamp used in the projection type liquid crystal display, a high-pressure mercury-vapor lamp is mainstream at present. However, it is quite difficult to control the optical output intensity with the high-pressure mercury-vapor lamp. Therefore, a method is desired in which the amount of incident light to the optical modulation device can be changed corresponding to the image signal, without changing the optical output intensity itself of the lamp.
Moreover, in addition to the above problems, since the brightness of a light source is fixed in the current projection type display, there are problems in that the screen becomes too bright, for example, in a dark viewing environment, and that when the projection screen size is changed by the projection distance or zooming of a projection lens, the brightness of the screen changes corresponding thereto.